In electronic devices based on semiconductors, the charge degree of freedom of carriers (electrons and holes) is utilised to functionalise the electronic properties of these devices. In addition, to the charge degree of freedom, electrons exhibit a spin degree of freedom. Recently, spintronics, which exploits the spin degree of freedom instead of or in addition to the charge degree of freedom of electrons, has attracted much attention as a result of it having potential applications in next generation information technology.
Upon using the charge as well as spin degrees of freedom simultaneously, spintronics aims at producing novel functionalities and properties of devices which cannot be achieved when using conventional electronics. A spin current gives rise to a variety of spintronic functionalities.
Energy dissipation of a spin current is low compared to that of a charge current. Hence, it can be expected that spintronic devices are potentially applicable for efficient energy transport. Thus the generation and detection of a spin current are indispensable when exploiting a spintronic device. Spin pumping has been proposed as a way to generate a spin current (Physical Review B 19, p. 4382, 1979). To detect a spin current, electrical detection using the inverse spin Hall effect (ISHE), which allows for converting a spin current into an electromotive force, has been proposed (Applied Physics Letters 88, p. 182509, 2006).
The Boltzmann equation in steady state is expressed asvk·∇ƒkσ+(eE/h)·∇kƒkσ=(∂ƒkσ/∂t)scatt  (1)
where the velocity of the electrons vk is expressed asvk=ℏk/m, 
E is the external electric field, ƒkσ is the distribution function of electrons with momentum k, mass m, charge e, and spin σ. On the right-hand side is an electron scattering term, the collision term due to impurity scattering. Here the scattering term on the right side of eq. (1) is given by the flowing eq. (2):∂ƒkσ/∂t)scatt=Σk′σ′[Pkk′σσ′ƒk′σ′−Pk′kσσ′ƒkσ]  (2)
where the left term is the scattering term from a state with momentum k′ and spin a′ to a state with the momentum k and spin a (and vice-versa for the right term), and wherePk′kσ′σ=(2π/ℏ)nimp|<k′σ′|{circumflex over (T)}kσ>|2δ(ξk−ξk′)  (3)
is the scattering probability from the state |k σ> to |k′σ′>. nimp is the impurity concentration and {circumflex over (T)} is the scattering matrix whose matrix elements are calculated within the second-order Born approximation (Pk′kσσ′ can be obtained by changing the position of the prime in eq. (3)). The solution of the Boltzmann equation yields the distribution function ƒkσ,
                              f                      k            ⁢                                                  ⁢            σ                          ≈                                            f              0                        ⁡                          (                              ξ                k                            )                                -                      σ            ⁢                                          ∂                                                      f                    0                                    ⁡                                      (                                          ξ                      k                                        )                                                                              ∂                                  ξ                  k                                                      ⁢                          δμ              ⁡                              (                r                )                                              +                                    τ              tr                        ⁢                                                                                ∂                                                                  f                        0                                            ⁡                                              (                                                  ξ                          k                                                )                                                                                                  ∂                                          ξ                      k                                                                      ⁡                                  [                                                            v                      k                                        -                                                                  α                        H                                            ⁢                                              σ                        σσ                                            ×                                              v                        k                                                                              ]                                            ·                              ∇                                                      μ                    σ                                    ⁡                                      (                    r                    )                                                                                                          (        4        )            
Where σ is the electrical conductivity, f0(ξk) is the Fermi distribution function of an electron with energy ξk, σσσ is the Pauli spin matrix, τtr the scattering time due to impurities (the transport relaxation time), μ(r) is the electrochemical potential. Here αH is the conversion coefficient (the dimensionless parameter of the skew scattering strength):αH=(2π/3)ηSON(0)Vimp  (5)whereηSO=kF2ηSO 
is the dimensionless spin-orbit coupling parameter, N(0) is the density of states of electrons at the Fermi level, and Vimp is the strength of the impurity potential. The electrochemical potential, more particularly the non-equilibrium spin accumulation, is expressed as andδμ(r)=(½)(μ↑(r)−μ↓(r))  (6)μ↑(r)=εF+eϕ+δμ(r) and μ↓(r)=εF+eϕ−δμ(r)  (6)
where the arrows define the electrochemical potential for up spin and down spin channels, respectively and εF and ϕ are the Fermi energy and electric potential, respectively, and ϕ=−∇E.
We obtain a transverse charge current generated by a spin current (jsx along direction x) generated along the out-of-plane direction,jsα=−(σx/e(∂δμ(r)/{circumflex over (α)}x)  (7)
through the ISHE as
                              σ          i                =                              -            2                    ⁢                                          ⁢                      e            2                    ⁢                                    ∑              k                        ⁢                                                  ⁢                                          τ                tr                            ⁢                                                ∂                                                            f                      0                                        ⁡                                          (                                              ξ                        k                                            )                                                                                        ∂                                      ξ                    k                                                              ⁢                                                (                                      v                    k                    i                                    )                                2                                                                        (        8        )            
where σi is the electrical conductivity along the i axis (i=x, y, z).
Charge current density jc converted from a spin current is expressed in eq. (9) below. The charge current density flowing along the y direction jcy rising from the spin current flowing along the x direction is expressed in eq. (10). Thus the charge current density jcy generated by the inverse spin Hall effect is expressed as jcy=αH·jsx. That is, using the distribution functionƒkσ and jc=eΣkvk(ƒk↑+ƒk↓),  (9)
we obtain
                              j          c          y                =                  2          ⁢                                          ⁢          e          ⁢                                    ∑              k                        ⁢                                                  ⁢                                          τ                tr                            ⁢                                                ∂                                                            f                      0                                        ⁡                                          (                                              ξ                        k                                            )                                                                                        ∂                                      ξ                    k                                                              ⁢                                                (                                      v                    k                    y                                    )                                2                            ⁢                              α                H                            ⁢                                                ∂                                      δμ                    ⁡                                          (                      r                      )                                                                                        ∂                  x                                                                                        (        10        )            
Hence, in order to enhance the charge current generated via the ISHE, it is important to use materials which exhibit a large conversion coefficient αH (see Physical Review B 19, p. 4382, 1979, and Applied Physics Letters 88, p. 182509, 2006).
The ISHE can generally be detected in the following materials: transition metals having either f- or 3d-orbitals, such as Pt, Au, Pd, Ag or Bi as well as alloys composed of these transition metals, or alloys composed of one of these transition metals and either Cu, Al or Si.
Amongst these metals and alloys, Pt- and Bi-doped Cu are very promising as their ISHE efficiencies are very high. When an external magnetic field is applied to such materials, an inverse spin Hall voltage EISHE is generated in a direction perpendicular to the external magnetic field.
Organic materials are considered promising for spintronics primarily because they can exhibit a long spin coherence time because there is only very weak spin-orbit interaction. However, because a strong spin-orbit interaction is considered important for the process of spin-to-charge conversion, organic materials have, hitherto, not been considered useful for this purpose.
Spintronic technology is promising to deliver a new generation of information processing, information storage and energy conversion devices with reduced power dissipation, higher integration density, high speed and increased energy efficiency. Most spintronic devices rely on sophisticated multilayer architectures with ultrathin, nanometer thick magnetic or non-magnetic films and carefully controlled heterointerfaces. They are typically deposited by sophisticated, vacuum based sputtering or molecular beam epitaxy and patterned by high-resolution lithography, such as deep-UV and electron-beam lithography. These are mostly compatible with existing process technologies for silicon integrated circuits and have a similar cost structure. The integration density is very high and the cost per individual spintronic device is very low, but the technology is expensive for applications that require bulk materials or large substrate areas. Examples of such large-area/bulk spintronic devices include thermoelectric devices that use the spin Seebeck effect to convert a temperature gradient into an electric voltage (see, for example, Bauer, G. et al., Spin caloritronics, Nat. Mat. 11, 3091 (2012).
These typically require bulk quantities of materials and thicker films to allow using a large temperature gradient in order to optimize the thermodynamic conversion efficiency of such a thermoelectric converter. Other examples are low-cost memory devices and intelligent labels that can be integrated into smart packaging for anti-counterfeiting, brand-protection or supply-chain management as well as smart surfaces that have memory elements distributed over relatively large substrate areas.
Conjugated polymers and small organic molecules are enabling new flexible, large-area, low-cost optoelectronic devices, such as organic light-emitting diodes, transistors and solar cells. Due to their exceptionally long spin lifetimes, these carbon-based materials could also have an important impact on spintronics, where carrier spins play a key role in transmitting, processing and storing information. However, to exploit this potential, a method for direct conversion of spin information into an electric signal is needed. The motion of electrons is coupled to their spins through the spin-orbit interaction, a fundamental relativistic effect which is essential for transforming spin information into an electric signal. The spin-orbit interaction causes a flow of spins, a spin current, to induce an electric field perpendicular both to the spin polarization and the direction of the spin current (see, for example, Maekawa, S., Valenzuela, S., Saitoh, E. & Kimura, T. (eds.) Spin Current (Oxford University Press, Oxford, 2012). This is called the Inverse Spin Hall Effect (ISHE). The ISHE has been important for exploring spin physics in metals and semiconductors, because it enables simple and versatile detection of spin currents in non-magnetic materials. The spin-orbit interaction is also an important mechanism for spin relaxation in solids; nonequilibrium spin polarization relaxes due to the coupling between spins and their motion. The spin lifetime in inorganic metals and semiconductors with strong spin-orbit coupling is typically short, typically less than 1 ns. In contrast, the spin lifetime in organic materials composed of light elements can be exceptionally long in excess of 10 μs. This is because the strength of the spin-orbit interaction scales with the fourth power of the atomic number and is relatively weak. Spin relaxation is also influenced by relatively weak hyperfine interactions. It is well established from such considerations that the spin relaxation times in organic materials can be very long, but it is widely believed that the weak spin-orbit interaction in these materials also implies that efficient spin-charge conversion and the inverse spin-Hall effect cannot be observed in organic materials.
We will describe organic electronic devices for spin-charge conversion which operate contrary to the prevailing belief/prejudice.
As outlined above, materials which exhibit a large conversion coefficient αH should be used in order to enhance the charge current using the ISHE. However, αH for conventional materials is of the order of 10% at its maximum, as demonstrated for Pt. Theories predict that αH for Bi-doped Cu could be of the order of 25%. Nonetheless, the conversion coefficient αH has its limitation.
This invention aims at enhancing the spin current to charge current conversion.